The present invention relates to a method for monitoring and controlling a slip-controlled brake system for automotive vehicles. The wheel rotational behavior is sensed and, on detection of an imminent locked condition, the braking pressure in the wheel brakes of the wheel concerned or in the wheel group to which the wheel tending to lock belongs will be temporarily kept constant or varied. That is, decreased and, if necessary, re-increased. The present invention also provides circuit configurations for implementing these methods.
As is known, slip-controlled brake systems prevent locking of the wheels even under unfavorable road conditions, in particular on snowy and icy roads, and upon panic stops by way of temporarily reducing the braking pressure or keeping it constant. These systems maintain the driving stability and the steerability of the vehicle, while attaining a short stopping distance, since the braking pressure is reduced just enough to prevent locking. According to the inherent principle, the braking pressure variation is temporarily kept constant or even decreased, on commencement of the slip control, as a function of the wheel rotational behavior, the transmissible brake torques, etc. Therefore, malfunctions of the controller will not only defeat the effect of the slip control, but may also cause dangerous situations, for instance due to an excessive reduction of the braking pressure, slow reaction, etc.
Arrangements and circuit configurations for monitoring slip-controlled brake systems are known. A minimum requirement to be fulfilled by these arrangements is to bring about at least that condition which would prevail without slip control upon the occurrence of an error. To this end, the slip control will be deactivated on detection of an error, and this condition will be signalled to the driver. In hydraulic brake slip control apparatus it is known to monitor the pressure in the hydraulic source and the fluid status or the fluid quantity in the hydraulic system and to partially or entirely deactivate the apparatus on the detection of defects, for example, by closing the pressure-reducing valves or by not releasing them (German patent application Nos. 32 32 051.5 and 32 32 052.3 which correspond to U.S. Pat. Nos. 4,523,791 and 4,555,144 respectively).
It is likewise known to monitor the valves inserted into the pressure fluid conduits from the braking pressure generator to the wheel brakes and from the wheel brake to a supply reservoir, which to control the constant braking pressure and the pressure reduction. The circuit configuration as disclosed in German patent specification No. 22 51 472 controls the mechanical movement of the solenoid valve armature from the measurement of the variation of the valve current. However, the valve monitoring is very complicated and relatively unreliable because the known circuit configurations only detect errors of a certain kind.
It is, therefore, an object of the present invention to increase the safety and the reliability of slip-controlled brake systems and to eliminate the danger of skidding which is caused by overbraking of the rear wheels as a result of a defective control. That is, while brake systems without slip control are devised such that the rear wheels come to a standstill only after the front wheels have locked, it is easily possible (for example, when a rear-wheel pressure-reducing valve fails or is jammed) that only the rear wheels will lock, but that the braking pressure at the front wheels will continue to be controlled. This driving condition would be extremely critical, because there would be a high risk of skiding for the vehicle due to the minor lateral guidance of the stationary rear wheels.